Effective reduction of EMI by spread spectrum clocking is shown in two patents for which the inventor of this invention is also an inventor. These are U.S. Pat. Nos. 5,488,627 and 5,631,920. Both of these patents show circuits in which the spread algorithm is stored by differing digits addressed in memory. Since the memories are read only (ROM) memories, the circuits disclosed are fixed into one mode of operation.
It is desirable that each circuit be readily variable so that the same hardware design may be produced in volume, yet the circuit may be varied in use, either radically for widely different requirements, moderately for switching to a low activity mode, or slightly to optimize the operation for otherwise identical operation.